The present invention relates to a laser marking apparatus, and more particularly, to a one-shot laser marking apparatus which uses a transmission type liquid crystal mask of a matrix form as a pattern mask for marking.
A conventional laser marking apparatus which uses a transmission type liquid crystal mask of a matrix form as a pattern for marking is explained with reference to FIG. 1 through FIG. 6.
FIG. 1 is a schematic block diagram for explaining the conventional laser marking apparatus. In the drawing, a pulse laser oscillator 1 as represented by, for example, a YAG laser enables to emit a pulse laser beam having a wavelength within a wavelength range from visible light to near infrared light. A linearly polarized laser beam 2 emitted from the pulse laser oscillator 1 is expanded by an expander 3 which is formed by combining a concave and a convex cylindrical lenses and is irradiated onto a liquid crystal mask 4. The liquid crystal mask 4 is operated by a drive and control unit 5.
Now the structure of the liquid crystal mark 4 is explained with reference to FIG. 2 which shows a part of the cross section of the liquid crystal mask 4. On an upper glass plate 14 scanning electrodes 15 for dot matrix are disposed and formed and on a lower glass plate 18 signaling electrodes 17 for dot matrix are also disposed and formed in a direction orthogonal to that of the scanning electrodes 15. The surface of the scanning electrodes 15 and the signaling electrodes 17 is applied with an orientation treatment and liquid crystal 16 is sandwiched between the upper glass plate 14 and the lower glass plate 18 to constitute a transmission type liquid crystal cell serving as the liquid crystal mask 4 of a matrix form.
FIG. 3 is a plane view of a part of the liquid crystal mask 4 thus constituted seen from above the upper glass plate 14. As seen from the drawing on the liquid crystal mask 4 a multiplicity of crossing portions 61 forming dot portions and of non-cross portions 62 forming clearances between the dots are formed by the scanning electrodes 15 and the signaling electrodes 17 for dot matrix.
FIG. 4 is a view for explaining the operation of the liquid crystal mask 4 when a character to be marked on an article is laterally turned "U". As seen from the drawing, information or signal voltages for displaying the character on the liquid crystal mask 4 are applied only to the dots in the matrix indicated by hatching 80 from the drive and control unit 5 and no signal voltages are applied to the other dots, in that a positive mask pattern is applied. The plane of polarization of laser beam 6 which has passed through the portion applied with no signal voltage of the liquid crystal mask 4 is caused to rotate by 90.degree. but the plane of polarization of the laser beam 6 which has passed the portion of the liquid crystal mask 4 applied with the signal voltage is caused no rotation because of the controlled optical property of the liquid crystal in this portion. A polarized beam splitter 7 is adapted to pass the laser beam 6 which has passed through the portion applied with the signal voltage as indicated by the hatching 80 in FIG. 4 and is adapted to reflect the laser beam 6 which has passed through the portion applied with no signal voltage. Laser beam 8 which has passed the polarized beam splitter 7 is reflected at a bend mirror 10, passes via a condenser lens 11 and is imaged on an article 12 to be marked to mark the character. Laser beam 9 which has been reflected at the polarized beam splitter 7 travels to an absorbing member 13 and is absorbed there.
FIG. 5 is a view for explaining heat diffusion of the laser beam 8 irradiated upon the article 12 to be marked and FIG. 6 is also a view for explaining the resultant marked area reduction due to the heat diffusion. As shown in FIG. 5 the heat caused by the irradiation of the laser beam 8 on the article 12 to be marked diffuses therein as indicated by allows 19, as a result, an effective marked area on the article 12 is reduced as indicated by hatched square areas 90 in FIG. 6 which is smaller than the actual laser beam irradiated area corresponding to the hatched area 80 in FIG. 4.
JP-A-64-11088(1989) of which corresponding U.S. Patent is U.S. Pat. No. 4,818,835 also discloses a laser marker similar to that explained above.
In the above conventional laser marking apparatus which uses a transmission type liquid crystal mask of a matrix form as a pattern mask for marking, no counter measured was taken for preventing diffusion of heat generated by irradiated laser beam toward the clearances between dots, such that effective marking area in a dot is reduced which results in non-sharp marking dots on the article to be marked, in particular on the article having a high heat conductivity.